66 BULLETIN 61, U. S. DEPAETMENT OF AGRICULTUEE. 



F. L. Hess* describes the occurrence of gypsum in recent lake beds in theMojave 

 Desert, CaL The beds occur in a periodic lake in the vicinity of Amboy, Cal. Bristol 

 Lake is the name given to the area. The gypsum occurs in the lake bottom close to 

 the shores of the lake. The bed is of variable thickness and its maximum thickness 

 has not been determined. In one place gypsum was found to a depth of 9.5 feet, the 

 iip])er layers being more or less mixed \v'\th dirt. A brine is reached in the lake bed 

 at a depth varying from 8 to 10 feet. Prospect holes show the deposit to be confined 

 from within one-half to 1 mile of the old shore line. The gypsum is of a granular nature. 

 Hess ascribes the localization of a deposit of this kind as being due to the greater 

 evaporation rate of the lake waters near the shore. Capillarity in the marginal material 

 also undoubtedly has contributed to the local concentration of the gypsum. 



Rock gypsum is being mined at Mound House, Arden, and Ludwig. The deposits 

 are of considerable commercial importance. The gypsite deposits at Mound House 

 were worked for a time, but have been idle for some years. They are of doubtful 

 value. The gypsum at Bristol Lake is reported by Hess as being exploited by the 

 Pacific Cement Plaster Co. 



CONCLUSION. 



Repeated reference has been made to the Stassfurt deposits of Germany in connec- 

 tion with the search for potash salts in the United States. WMle this has served a 

 useful purpose in stimulating the search for salines, it perhaps has resulted in the 

 opinion that similar deposits might be expected in the Great Basin. Such a view 

 can not now be held. The German deijosits are in the Triassic and they, as well as 

 the associated sedimentaries, have been folded and tilted. They represent complete 

 "desiccation and more or less secondary action before, during, and after tectonic dis- 

 turbance. Omitting from present consideration the deposits of the Jurassic and 

 Tertiary, the saliniferous deposits of the Great Basin may be said to represent com- 

 paratively recent geologic activity. They are confined to the Quaternary lake and 

 desert basins. The older deposits were formed in earlier periods of desiccation, but 

 desiccation did not reach extreme conditions. The present deposits are in process 

 of formation. Very little disturbance of the Quaternaiy and recent sedimentaries has 

 taken place. More or Less secondary action, such as solution, recrystallization, and 

 movement of brines, is taking place. It may be said that the basin deposits already 

 discovered represent the initial stages of what in time might result in deposits rather 

 remotely similar to Stassfurt, but of much less magnitude. 



The influence of regional rocks has been commented upon and the prevalence of 

 volcanics in the Great Basin has caused geologists to turn to this region as a place in 

 which to look for potassium salts. Regional differences, caused by the prevalence 

 of different types of rocks, are manifest in the presence of alkali carbonates and 

 borates in the .western part of the Great Basin and the presence of chlorides in the 

 eastern portion where sedimentaries predoniinate . In the case of potassium, no 

 such marked difference is shown. The potassium content in the saline residue of the 

 water of Great Salt Lake is not much less than that of Mono, Owens, and Pyramid 

 Lakes. Humboldt Lake, North, Middle and South Alkali Lakes, it is true, show a 

 higher content of potassium, but these are relatively unimportant. The resistance 

 to -weathering of the potash-rich minerals and the ease with which this element is 

 absorbed and removed from surface and underground waters might well account for 

 the low content of potash in all of the lakes. 



With the exception of the crusts and efflorescences about hot springs and in soils, 

 no notably high potassium content has been reported from saUnes taken from beds. 

 The potas.'^ium content in. material of this nature ranges from less than ] to 2 per cent. 

 It is not in the salts which have crystallized out, but in the residual brines or mother 

 liquors that concentration of potassium has taken place, and it is to these that we 

 must look for potassium salts. As desiccation approaches completion, so will the 

 residual brines increase in proportion of potassium. A near approach to complete 

 desiccation would give a brine high in potash. The fortuitous absorption and sealing 

 over of such a brine would protect it from further changes, except those produced by 

 circulating underground waters. It is evident that the above action might occur at 

 different stages of desiccation, and brines varying in degree of potash content would 

 be absorbed and sealed in the same way. Sealing would not necessarily have to be 

 caused by the formation of impervious layers, although tliis would be more effective 

 than a layer of sand. A layer of sand, subsequent ly flooded with water, would deplete 

 by diffusion the partially concentrated brine beneath and, in time, a much weaker 

 brine would result. It should be noted that the abi-orption of saline waters which 



1 Bui. No. 413, U. S. Geol. Survey, p. 12o. 



